1. Field of the Invention
The present invention relates to a GaN crystal substrate used in a semiconductor device and the like, and particularly to a GaN crystal substrate of which front and rear surfaces are distinguishable from each other.
2. Description of the Background Art
A GaN crystal substrate is widely used as a substrate for a semiconductor device such as a light emitting device, an electronic device, or a semiconductor sensor. Here, by forming at least one semiconductor layer on a surface of a GaN crystal substrate on which a crystal is grown (also referred to as a crystal growth surface or front surface, hereinafter the same applies), a semiconductor device is formed. Accordingly, in order to manufacture a semiconductor device, it has been necessary to distinguish between the front surface and rear surface (which is a surface opposite to the crystal growth surface, hereinafter the same applies) of a GaN crystal substrate.
By dividing a wafer that includes a GaN crystal substrate and at least one semiconductor layer formed on the front surface thereof along a prescribed crystal orientation, each semiconductor device chip can be obtained. Accordingly, it has also been necessary to identify crystal orientation in order to manufacture a semiconductor device.
In order to distinguish between the front and rear surfaces of such a GaN crystal substrate, it has conventionally been necessary to form at least two orientation flats. In order to identify the crystal orientation of a GaN crystal substrate, at least one orientation flat has been formed in parallel with a specific orientation in the substrate (for example, orientation <1-100>) (for example, see Japanese Patent Laying-Open No. 2002-356398).
However, when a semiconductor layer is epitaxially grown on a surface on which crystal is grown of a GaN crystal substrate having such orientation flats formed, the morphology of the semiconductor layer is impaired, e.g., the thickness of the grown semiconductor layer varies between the center of the substrate and a portion near the orientation flats. Although a GaN crystal substrate without such orientation flats does not involve such a problem of impaired morphology of the grown semiconductor layer, the front and rear surfaces cannot be distinguished from each other and/or the crystal orientation cannot be identified.
Accordingly, there has been a desire for development of a GaN crystal substrate of which front and rear surfaces are distinguishable from each other and/or in which the crystal orientation can be identified, without impairing the morphology of a semiconductor layer grown on the GaN crystal substrate.